It is well known that hydrogen bonding between different molecules and between portions of the same molecule in biologically active substances is important in determining the specific reactivities of these compounds, and in explaining these properties. Such compounds include proteins, viruses, enzymes (as opposed to non-enzymatic proteins), antibiotics, drugs, etc. Although there have been some notable past successes of the formulation of molecular structures in which hydrogen bonding modes were an essential part, such as those for polypeptides (Pauling and Corey) and for DNA (Watson and Crick), some aspects of this important structural chemical property warrant further investigation. These are: C-H ...O systems, "symmetrical" hydrogen bonds, the requirement of complete hydrogen bonding, and strain in hydrogen bonds. It is proposed to carry out crystal structure determinations, by x-ray diffraction, of substances in which these features are present. It is also proposed to analyze structures already published in order to add to the understanding of these features.